As is known in the art, dielectric covercoats have been used to cover exposed conductors on a surface of a printed circuit board. The mask will confine the flow of solder to terminal and pad areas thus avoiding so-called solder bridging between pads and circuit lines, for example. Dielectric covercoats improve circuit performance and reliability by retarding electromigration and other forms of growth which may develop on conductors.
As is also known, conventional covercoats are typically provided from adhesive/polyimide or epoxy based soldermask materials. Adhesive coated polyimide covercoats have been used in the past but are expensive to process. For example, an adhesive coated polyimide sheet must have openings drilled and punched therein, and must then be aligned and thermally bonded over the exposed conductors of the circuit. Epoxy soldermasks lack flexibility and fail to provide optimum performance characteristics. Furthermore, conventional epoxy based solder masks tend to fracture when used in flex type circuits when the flex circuit is flexed.
Furthermore, when sheet adhesive systems are used to bond the circuit to a thermal plane, for example, the processing steps include heating, and applying pressure to provide a satisfactory bond between the flex circuit and the thermal plane.
Other materials such as liquid polyimide and urethane for example, have in the past been proposed for use as soldermasks. These materials however have adverse effects on the printed circuits, require increased processing complexity, are difficult to apply, require high temperature curing and are expensive.
The use of silicone covercoats have in the past been specifically avoided in printed circuit applications due to outgassing of the silicone. Silicone by-products contaminate the exposed surfaces of the circuit and degrade adhesion and solderability. Thus, silicone has generally not been considered useful as a covercoat or even an adhesive in printed circuit applications.
It would, however, be desirable to provide a printed circuit assembly having a compliant covercoat which may be easily disposed over the printed circuit and which may provide the printed circuit having low moisture absorption properties.